This invention relates to materials useful for enhancing the browning and crispening and/or for providing uniform heating of foods cooked in microwave ovens.
Food preparation and cooking by means of microwave energy has, in recent years, become widely practiced as convenient and energy efficient. Microwave cooking of precooked and uncooked food products has traditionally produced bland-appearing and soggy meats and pastry goods. To alleviate this problem and aid the browning and crispening of the surface of a cooked food item, there have been developed a number of packaging materials specially adapted for use in microwave cooking. Many such known packaging materials incorporate a microwave susceptor material, i.e., a material capable of absorbing the electric or magnetic portion of the microwave field energy to convert that energy to heat. The susceptor material generally heats the adjacent surface of the food by conduction to a sufficiently high temperature to crispen or scorch the surface while direct microwave exposure of the food heats the interior.
U.S. Pat. No. 4,267,420 to Brastad discloses a packaging material which is a plastic film or other dielectric substrate having a thin semiconducting coating. A food item is wrapped in the coated film so that the film conforms to a substantial surface portion of the food item. On exposure to microwave energy, the film converts some of that energy into heat which is transmitted directly to the surface portion so that a browning and/or crispening is achieved.
U.S. Pat. No. 4,518,651 to Wolfe discloses flexible composite materials exhibiting controlled absorption of microwave energy comprising a porous dielectric substrate coated with electrically conductive particles, such as particulate carbon, in a thermoplastic dielectric matrix. The porous substrate is a sheet or web material, usually paper or paperboard.
U.S. Pat. No. 4,434,197 to Petriello et al. discloses a flexible multi-layer structure having at least one layer colored with a pigment and/or energy absorber with the outer two layers consisting of pure polytetrafluoroethylene to provide a food contacting surface. Disclosed as suitable energy absorbers are colloidal graphite, carbon and ferrous oxide.
U.S. Pat. No. 4,230,924 to Brastad et al. discloses a flexible wrapping sheet of dielectric material, such as polyester or paperboard, capable of conforming to at least a portion of the shape of a food article, and having a flexible metallic coating thereon. The coating, e.g., of aluminum, chromium, tin oxide, silver or gold, converts a portion of microwave energy into thermal energy so as to brown or crispen that portion of the food adjacent thereto.
The above-mentioned patents are only a few of the many patents disclosing the use of susceptor materials to aid in the browning and crispening of foods cooked by microwave energy. One problem which the known art does not address is the uneveness of cooking and/or browning that can occur in a microwave oven as a result of position-to-position electric and magnetic field strength variations in the oven. These field strength variations derive from an assortment of standing waves at the different modal wavelengths that exist in the enclosed oven cavity due to the interference between forward-going and reflected waves. The locations of these interferences (or "hot" or "cold" spots) vary not only between ovens of different manufacturers, but vary between ovens of the same manufacturer and by the position, type, and amount of food exposed to the electromagnetic fields. Also, as microwave ovens become smaller and more compact, the problems of uneveness will increase since the number of standing wave modes available for averaging are less and the microwave power density in the oven cavity becomes higher. Therefore, the difference between the "hot" spot and the "cold" spot temperatures become more intense in a smaller cavity.
It is therefore an object of this invention to ameliorate the inherent uneveness of response to the electric and magnetic fields in a microwave oven and to provide a composite susceptor material capable of affording uniform and consistent heating and browning of a food item by microwave energy independent of its placement within the microwave oven.